The present invention relates generally to implantable metallic medical devices. More specifically, the present invention relates to implantable medical devices, including, for example, surgical and endoluminal vascular grafts, stent grafts, covered stents, skin grafts, shunts, bone grafts, surgical patches, non-vascular conduits, valvular leaflets, filters, occlusion membranes, sphincters, artificial tendons and ligaments. More specifically, the present invention relates to implantable medical grafts fabricated of metallic or pseudometallic films of biocompatible materials and having a plurality of pleats or corrugations in the film. Where the graft is of a generally tubular shape, the pleats or corrugations are preferably circumferential and axially positioned along the longitudinal axis of the graft. In accordance with a preferred embodiment of the invention, the inventive medical grafts may have a plurality of microperforations passing through the film. The plurality of microperforations may serve multiple purposes, including, for example, permitting geometric deformation of the film, imparting a fabric-like quality to the film, imparting flexibility to the film, permitting tissue ingrowth and promoting healing. The term “fabric-like” is intended to mean a quality of being pliable and/or compliant in a manner similar to that found with natural or synthetic woven fabrics. The inventive medical grafts may have both perforated and imperforated regions along at least portions of the surface area of the graft.
The inventive implantable grafts are fabricated entirely of self-supporting, coherent films made of biocompatible metals or biocompatible pseudometals. Heretofore in the field of implantable medical devices, it is unknown to fabricate an implantable medical device that comprises a graft as at least as one of its elements, such as a stent graft or covered stent, fabricated entirely of self-supporting, coherent metal or pseudometal materials. As used herein the term “graft” is intended to indicate any type of device or part of a device that comprises essentially a material delimited by two surfaces where the distance between said surfaces is the thickness of the graft and exhibits integral dimensional strength to be self-supporting such that it is capable of use in vivo without the need for an ancillary supporting structure, such as a stent or other structural reinforcement to maintain an enlarged in vivo diameter. The inventive graft may have microperforations that pass through regions of the thickness of the graft such that the graft has imperforate regions and microperforate regions, or may have microperforations through all wall surfaces of the graft or be completely imperforate. The inventive grafts may be formed in planar sheets, toroids, and in other shapes as particular applications may warrant. However, for purposes of illustration only, the present application will refer to tubular grafts. For purposes of this application, the terms “pseudometal” and “pseudometallic” are intended to mean a biocompatible material which exhibits biological response and material characteristics substantially the same as biocompatible metals. Examples of pseudometallic materials include, for example, composite materials and ceramics. Composite materials are composed of a matrix material reinforced with any of a variety of fibers made from ceramics, metals, carbon, or polymers.
When implanted into the body, metals are generally considered to have superior biocompatibility than that exhibited by polymers used to fabricate commercially available polymeric grafts. It has been found that when prosthetic materials are implanted, integrin receptors on cell surfaces interact with the prosthetic surface. The integrin receptors are specific for certain ligands in vivo. If a specific protein is adsorbed on a prosthetic surface and the ligand exposed, cellular binding to the prosthetic surface may occur by integrin-ligand docking. It has also been observed that proteins bind to metals in a more permanent fashion than they do to polymers, thereby providing a more stable adhesive surface. The conformation of proteins coupled to surfaces of most medical metals and alloys appears to expose greater numbers of ligands and preferentially attract endothelial cells having surface integrin clusters to the metal or alloy surface relative to leukocytes. Finally, metals and metal alloys exhibit greater resistance to degradation of metals relative to polymers, thereby providing greater long-term structural integrity and stable interface conditions.
Because of their relatively greater adhesive surface profiles, metals are also susceptible to short-term platelet activity and/or thrombogenicity. These deleterious properties may be offset by administration of pharmacologically active antithrombogenic agents in routine use today. Surface thrombogenicity usually disappears 1-3 weeks after initial exposure. Antithrombotic coverage is routinely provided during this period of time for coronary stenting. In non-vascular applications such as musculoskeletal and dental, metals have also greater tissue compatibility than polymers because of similar molecular considerations. The best article to demonstrate the fact that all polymers are inferior to metals is van der Giessen, W J. et al. Marked inflammatory sequelae to implantation of biodegradable and non-biodegradable polymers in porcine coronary arteries, Circulation, 1996: 94(7):1690-7.
Normally, endothelial cells (EC) migrate and proliferate to cover denuded areas until confluence is achieved. Migration, quantitatively more important than proliferation, proceeds under normal blood flow roughly at a rate of 25 μm/hr or 2.5 times the diameter of an EC, which is nominally 10 μm. EC migrate by a rolling motion of the cell membrane, coordinated by a complex system of intracellular filaments attached to clusters of cell membrane integrin receptors, specifically focal contact points. The integrins within the focal contact sites are expressed according to complex signaling mechanisms and eventually couple to specific amino acid sequences in substrate adhesion molecules. An EC has roughly 16-22% of its cell surface represented by integrin clusters. Davies, P. F., Robotewskyi A., Griem M. L. Endothelial cell adhesion in real time. J. Clin Invest. 1993; 91:2640-2652, Davies, P. F., Robotewski, A., Griem, M. L., Qualitiative studies of endothelial cell adhesion, J. Clin. Invest. 1994; 93:2031-2038. This is a dynamic process, which implies more than 50% remodeling in 30 minutes. The focal adhesion contacts vary in size and distribution, but 80% of them measure less than 6 μm2, with the majority of them being about 1 μm2, and tend to elongate in the direction of flow and concentrate at leading edges of the cell. Although the process of recognition and signaling to determine specific attachment receptor response to attachment sites is incompletely understood, availability of attachment sites will favorably influence attachment and migration. It is known that materials commonly used as medical grafts, such as polymers, do not become covered with EC and therefore do not heal after they are placed in the arteries. It is therefore an object of this invention to replace polymer grafts with metal grafts that can potentially become covered with EC and can heal completely. Furthermore, heterogeneities of materials in contact with blood flow may be controlled by using vacuum deposition techniques to form the materials to make the inventive grafts.
There have been numerous attempts to increase endothelialization of implanted medical devices such as stents, including covering the stent with a polymeric material (U.S. Pat. No. 5,897,911), imparting a diamond-like carbon coating onto the stent (U.S. Pat. No. 5,725,573), covalently binding hydrophobic moieties to a heparin molecule (U.S. Pat. No. 5,955,588), coating a stent with a layer of blue to black zirconium oxide or zirconium nitride (U.S. Pat. No. 5,649,951), coating a stent with a layer of turbostatic carbon (U.S. Pat. No. 5,387,247), coating the tissue-contacting surface of a stent with a thin layer of a Group VB metal (U.S. Pat. No. 5,607,463), imparting a porous coating of titanium or of a titanium alloy, such as Ti—Nb—Zr alloy, onto the surface of a stent (U.S. Pat. No. 5,690,670), coating the stent, under ultrasonic conditions, with a synthetic or biological, active or inactive agent, such as heparin, endothelium derived growth factor, vascular growth factors, silicone, polyurethane, or polytetrafluoroethylene, (U.S. Pat. No. 5,891,507), coating a stent with a silane compound with vinyl functionality, then forming a graft polymer by polymerization with the vinyl groups of the silane compound (U.S. Pat. No. 5,782,908), grafting monomers, oligomers or polymers onto the surface of a stent using infrared radiation, microwave radiation or high voltage polymerization to impart the property of the monomer, oligomer or polymer to the stent (U.S. Pat. No. 5,932,299). However, these approaches fail to remedy the lack of clinically acceptable endothelialization of polymer grafts.
It is, therefore, desirable that the inventive graft be fabricated entirely of metallic and/or pseudometallic materials wherein at least a portion of the wall surfaces of the graft have a plurality of pleats or undulations to provide a self-supporting, stand-alone graft that exhibits a greater capacity for endothelialization than that of conventional polymeric grafts.